Research and development on magneto-optical disks have been made as being rewritable optical disks, and some of the magneto-optical disks have been already practically used as external memory designed for computers.
In the magneto-optical disk, a magnetic thin film with perpendicular magnetization is used as a recording medium, and a light is used in recording and reproducing. Thus, compared with a floppy disk or a hard disk wherein a magnetic thin film with in-plane magnetization is used, the magneto-optical disk has larger recording capacity.
However, since recording density of the magneto-optical recording medium is determined by a size of the light beam used in recording and reproducing on the recording medium, there is a limit in increasing memory capacity.
More specifically, when the size of the recording bit and the interval between the recording bits are smaller than the light beam spot diameter, a plurality of recording bits exist including the adjoining recording bits in the light beam spot, which increases noise, thereby presenting the problem that the recording bits cannot be reproduced separately.
In order to increase the recording density, the diameter of the light beam spot can be made smaller by making shorter the wavelength of the laser (light source), or by making larger the number of aperture (NA) of the objective lens so as to have greater angle of conversion.
In order to produce a laser with a shorter wavelength, a semiconductor laser designed for producing a short wavelength laser has been researched and developed. However, the semiconductor laser of this type presents the problem that an output light intensity is too low to be used as a light source for recording and reproducing on and from the magneto-optical disk.
On the other hand, when adapting a method of making larger the NA, it is necessary to maintain the optical axis of the light beam to be converged onto the disk in a perpendicular direction to the surface of the magneto-optical disk. Otherwise, the light beam spot diameter on the recording medium becomes larger on the contrary. Namely, when the NA is made larger, the precision in assembling optical system of the magneto-optical disk device or the restitution of the magneto-optical disk must be more strictly controlled than the conventional model, in order to prevent the problem that the light beam spot diameter becomes larger.
Therefore, at present, a wavelength of the semiconductor laser used in the magneto-optical disk is set in a range of 780-830 nm, and the NA of the objective lens is set in a range of 0.45-0.55.
Accordingly, a light beam spot diameter on the recording medium is set in a range of 1.7-2.0 .mu.m.
In accordance with the light beam spot diameter, a truck pitch of the magneto-optical disk, i.e., an interval between the recording bits in a radial direction of the magneto-optical disk is set in a range of 1.4-1.6 .mu.m.
If the light beam spot diameter is made smaller than the above, it is necessary to prevent the crosstalk which disturbs information recorded on the adjoining trucks. In order to prevent the crosstalk, a compensating circuit for carrying out a special waveform processing is required to be separately provided, thereby presenting the problem that the magneto-optical disk device becomes complicated.
In the case of carrying out an overwriting operation by the magnetic field modulation onto the magneto-optical disk, in order to obtain a sufficient size of the magnetic field, the magnetic field generation mechanism is required to be set close to the magneto-optical disk. Moreover, the magnetic field cannot be modulated at high speed.
In order to counteract the above problems, as disclosed in the Japanese Laid Open Patent Publication No. 62-175948 (Tokukaisho 175948/1987), an overwriting method by the light intensity modulation in which using a magneto-optical recording medium of a double layer structure of a recording layer composed of a magnetic thin film with perpendicular magnetization and a recording subsidiary layer, an overwriting operation is carried out by modulating only the laser power has been proposed.
In the above overwriting method by the light intensity modulation, when carrying out an overwriting operation, the magnetization direction of the recording subsidiary layer also changes. Thus, whenever an overwriting operation is to be carried out, it is necessary to arrange the magnetization direction of the recording subsidiary layer. When adapting the above method, not only the recording magnetic field generating mechanism, an initialization-use magnetic field generating mechanism is also required, thereby presenting the problems that the magneto-optical disk device becomes larger in size and that the manufacturing costs thereof increases.